Why did Charles Darwin go atheist?

First of all, Charles Darwin said he was not an atheist but an agnostic. He started out training to be a doctor, like his father; but he found the operations - in those days without anasthetic - too much to bear and left medical school. It was something of a tradition that failed doctors became priests and Darwin did study divinity before his trip on the Beagle which led to his discovery of evolution. Rather surprisingly, it wasn't the attacks by the Church on Darwin over his theory of evolution by natural selection, but the death of his daughter Annie in 1851 at the age of 10 that made him lose his Christian faith. He described the loss of faith as an ebbing away, rather than as a sudden change in belief.